Lithium secondary batteries using a compound represented by LiCoO.sub.2 containing a transition metal and lithium as a cathode have been proposed as power sources for various electronic instruments. This is because of their high working potential of from 3 to 4 V and high energy density as compared with conventional nickel-cadmium batteries or nickel-hydrogen batteries. In particular, accompanying the abrupt growth of portable electronics, (e.g., portable phones, note-type personal computers) there is a keen demand for rechargeable compact lithium secondary batteries which can be used to power these instruments. Furthermore, taking into account the present demand for electric power, it is forecast that an electric power storing apparatus for domestic use will be indispensable in the future. Furthermore, the lithium secondary battery is expected to be a candidate therefor.
A lithium secondary battery can have elevated electromotive force, energy density and reversibility, as compared with conventional secondary batteries, by using lithium as a guest in the insertion reaction. This is because lithium has the highest charge density and lowest redox potential among solids. However, for use in practical applications, problems such as elongation of life and fabrication of a combined battery must be solved, and a key therefor is the development of a cathode material having higher capabilities.
Among cathode materials, lithiated manganese oxides provide a higher battery voltage than commercially available LiCoO.sub.2 and in turn, high energy density. Moreover, manganese is more abundant than cobalt and is inexpensive. Accordingly, lithiated manganese oxides are the most promising cathode electroactive materials for the next generation of lithium batteries.
Several kinds of lithiated manganese oxides have been reported, however, only a spinel structure compound (LiMn.sub.2 O.sub.4) is used at present as the cathode material for lithium secondary batteries. LiMn.sub.2 O.sub.4 is drawing attention as a substitute material for LiCoO.sub.2 because it is inexpensive, however, this compound has a theoretical capacity of 148 mAh/g which is far smaller than the theoretical capacity of LiCoO.sub.2.
On the other hand, LiMnO.sub.2 has a layered rock-salt structure which is the same as that of LiCoO.sub.2, and has an electric capacity (284 mAh/g) almost equal to that of LiCoO.sub.2. Therefore, LiMnO.sub.2 is a promising substitute material. However, this compound is very difficult to synthesize by a typically used solid phase reaction method, and pure LiMnO.sub.2 can only be obtained by a molten salt method or under controlled conditions in argon. The reason therefor is that because LiMnO.sub.2 is present in the boundary region between a layered rock-salt structure and an irregularly arrayed rock-salt structure, a layered rock-salt structure in a semi-stable phase can be difficult to obtain.